Guys, this is not a simple subject... Entire Doctoral Thesis have been
written on this subject... Here's a few references for rumination..
Page three of the first reference has a graph of strength versus time
that is immediately understandable...
I didn't bother to get the url shortened, so it splits into two
lines... If you are unable to cut and paste the url together then the
graph probably won't excite you much either...

Or you can google... My search terms were < concrete strength versus
time >
I know that my understanding of concrete has increased by skimming
these articles..